This invention relates to conductive polymer compositions which are used to detect the presence of toxic substances in the environment. The conductive polymers of this invention are intrinsically conductive polymers which are conductive without the necessity of doping. These conductive polymers can be used in sensors and incorporated in dosimeters to selectively detect the presence of extremely toxic substances in the environment, such as chemical warfare agents.
Accurate selective detection and quantification of toxic substances is generally achievable at relatively high cost. Paper tape sensors coated with a compound which changes color when exposed to a toxic substance can be relatively inexpensive. However, the tape coating layer may respond to a number of discrete substances, making selectivity difficult. Since many substances are present in the environment in trace amounts, the potential for interference or false readings with such devices is significant.
Another category of relatively inexpensive chemical sensors is solid state devices, such as metal oxide films, which display a conductivity change in the presence of certain toxic chemical species, but usually at elevated temperatures. Although adequate for the gross detection of certain chemicals, these metal oxide sensors typically respond to a variety of substances, leading to problems with accuracy and selectivity. In addition, since the devices operate at elevated temperatures, they require a power source for heating and are subject to degradation.
The sensing techniques which have the required sensitivity and selectivity are typically extremely sophisticated, expensive scientific instruments which need to be operated by a skilled technician. For example, mass spectrometers are very sensitive and can achieve the desired selectivity, unless two or more species present in the sample give rise to ions with the same mass to charge ratio. Spectroscopic techniques in principle can achieve the desired selectivity and sensitivity provided that the chemicals do not overlap in terms of their spectroscopic features. However, such instruments typically cost tens of thousands of dollars.
Recent developments in chemical detection involve electronic xe2x80x9cnoses.xe2x80x9d These devices are described as being capable of detecting the presence of vapors including methanol, acetone, benzene, and toluene. Such devices are described in U.S. Pat. Nos. 5,698,089; 5,571,401; 5,788,833; and 5,891,398. The devices of these patents relate to chemical sensors, arrays of sensors, and sensor manufacturing techniques. The sensor contains a resistor element which is composed of a nonconductive organic polymer and a conductive material. When the analyte contacts the resistor element, the resistance changes (presumably due to a swelling of the substrate), and this change in resistivity is measured by an electronic signal measuring device. The conductive material can be a conductive polymer, such as doped polyaniline or polypyrrole, and these materials can be incorporated in a portable hand-held electronic device with an LCD read out. The devices described in these patents appear to operate at or near the percolation threshold of the polymer such that the resistivity of the polymer is changed due to the absorption of volatile organic compounds by the polymer matrix.
The use of conductive polymers for fabricating chemical sensors is a relatively recent technological advance. U.S. Pat. Nos. 5,145,645 and 5,310,507 relate to dosimeters for the selective detection of chemical species. The dosimeters include a doped conductive polyaniline which detects the presence of a specific chemical substance by an irreversible change in resistance of the polyaniline. The resistance is measured and displayed in an electronic device connected to the sensor. The individual chemical is identified by correlating the initial polymer resistance with the altered resistance, and comparing this difference with a calibration curve of known chemical entities. Typical chemical entities which may be detected using this approach include hydrazine and ammonia.
Polyaniline is a conductive polymer which has been extensively investigated, both in terms of its properties and potential uses. U.S. Pat. No. 5,624,605 relates to high molecular weight, plasticized, conductive polyaniline compositions. The conductivity of these compositions is provided by adding a functionalized protonic acid to the polyaniline. U.S. Pat. No. 5,378,402 relates to polymeric dopants for enhancing the conductivity of polymers, such as polypyrrole and polyaniline. Sulphonated polystyrene is an example of a negatively charged polymer which is suitable as a dopant. U.S. Pat. No. 5,227,092 is directed to electrically conductive polyaniline block copolymers which are doped using para-toluenesufonic acid as a sulfonated dopant.
U.S. Pat. No. 5,519,147 relates to polythiophene derivatives which contain crown ether moieties covalently bound to the 3 position of adjacent thiophene units. The modified polythiophenes can be used to measure the presence of chemical species, such as metal ions and organic molecules, in a reversible manner. The modified polythiophenes can be used to prepare sensors by coating the polymer onto an insulating substrate, and measuring the difference in conductivity of the polymer in the presence and absence of the species to be detected.
Recent incidents of terrorism around the world have underscored the need for devices for detection of chemical warfare agents (xe2x80x9cCW agentsxe2x80x9d) for use by both military and civilian personnel. Despite this need, the sensitive and chemically specific detection of CW agents remains a significant challenge. This is due, in part, to the extreme toxicity of CW agents: the median lethal dose for the nerve agent VX is 7 xcexcg per kg of body weight for a normal adult, requiring sensitivity levels in the range of parts per billion. Furthermore, to provide maximum effectiveness, CW agent sensors should preferably be used in a badge format that can be worn by each individual to ascertain exposure. This portability requirement places additional constraints on sensor design: the device should be low-cost, have a small form-factor (credit card size), and use very little power (e.g., battery-operated). In addition, it would be useful for a sensor system to have characteristics which are applicable to both military and commercial product markets. Satisfying these demanding requirements necessitates a novel approach to the measurement of low concentrations of CW agents.
It will be readily appreciated that a need exists for a compact dosimeter for the rapid and accurate detection of extremely toxic substances in the environment.
The present invention features a device and method for detecting the presence of toxic substances in the environment. The device of this invention can incorporate an array of individual sensing elements or sensors which are configured and designed to detect one or more specific toxic substances in the environment. The individual sensors are prepared from conductive polymers which are inherently conductive without the use of a dopant. The conductive polymers are included in a polymeric composition, which also includes at least one additive which reacts with a toxic substance to be detected. The reaction results in a change in a detectable characteristic of the polymeric composition which can be measured and displayed by the detector. Preferably, the change in the detectable characteristic is a change in conductivity of the composition.
Accordingly, in one aspect, inherently conductive polymers are provided for use in sensors for detecting toxic substances in the environment. The inherently or intrinsically conductive polymers do not require the use of dopants to achieve acceptable and measurable levels of conductivity, and, in fact, the use of a doping agent may render the polymers less effective. Polymers which are preferred in this invention include the substituted polythiophenes, such as regioregular poly (3-hexylthiophene) (xe2x80x9crrP3HTxe2x80x9d). Regioregular polythiophenes are particularly preferred, and such polymers are described in McCullough et al., J. Am. Chem. Soc., 115, pages 4910-4911 (1993), the pertinent disclosure of which is incorporated herein by reference. The regioregular orientation of the polymer is preferably head-to-tail. The regioregular polymers are semi-crystalline in structure, and the physical distortion of the polymer, such as a torsional distortion or conformational change, results in a change in conductivity. As used herein, and unless otherwise indicated, the term xe2x80x9cconductive polymerxe2x80x9d is intended to denote such inherently conductive polymers.
In another aspect, this invention includes polymeric compositions which are prepared from the conductive polymers and at least one additive. The additive is a substance which is permanently or irreversibly reactive with the CW agent to be detected. Such additives can be physically incorporated into the polymer by blending, copolymerized with the conductive polymer, or reacted with a pendant group of the polymer. Preferably, the additives are antidotes for the CW agents, such as syn-2-pyridine aldoxime (xe2x80x9coximexe2x80x9d) and 2-pralidoxime chloride (xe2x80x9c2-PAMxe2x80x9d). The additives interact with the CW agent, and cause distortion of the polymer, resulting in a change in conductivity.
In a further aspect of this invention, the polymeric composition can be used to prepare sensors. In addition to the polymeric composition, the sensors include a means for measuring a change in the detectable characteristic of the polymer in the presence of a toxic substance. The change in detectable characteristic is preferably a change in the conductivity or resistivity of the composition. However, other detectable changes which can be measured include changes in optical properties, changes in mass, and changes in capacitance or impedance. These changes can be measured using standard devices and instrumentation. For instance, surface acoustic wave devices or piezoelectric devices, such as quartz microbalances, for measuring the change of mass, UV, visible light or passive IR absorption for measuring a change in optical properties, comparators for electrical resistance measurement, and devices for measuring the reaction products resulting from the reaction of the CW agent and the additive. These measurement means are not intended to be exclusive, and more than one type of measurement can be made in response to one or more toxic substances.
In one particularly preferred embodiment of this aspect of the invention, the detectable change is a change in the resistivity of the sensor, and the polymeric composition is applied as a film to a suitable substrate material. In this embodiment, a thin film of the polymeric composition is applied to a suitable substrate material, preferably a non-conductive substrate, such as glass, a ceramic, or a non-conductive polymer. The sensor can incorporate electrical contact points or electrodes for connecting the sensor to a voltage or current source, or to other sensors to form an array of two or more of such sensors. The polymeric composition used to prepare an individual sensor can also include more than one additive component, with each additive being reactive with a single or different CW agent, provided that the CW agents are all part of a single class of such agents such that an individual sensor can respond to a variety of CW agents within a single class. The sensor array can include a plurality of individual sensors which may be responsive to a variety of CW agents in multiple chemical classes. Preferably, the sensor array comprises at least two individual sensors and is capable of responding to three classes of CW agents, i.e. the G-class of agents, such as GA, GB and GD; the H-class of agents, such as HD; and the V class of agents; such as VX. These agents typically have sufficient volatility to be detected as airborne toxins. The toxic substances of this invention can include substances other than CW agents, such as pesticides. This capability provides broad versatility to the detection device. The sensor array can also include individual sensors prepared from a diversity of conductive polymers, including conductive polymers which are not inherently conductive and may require the use of a dopant.
Another aspect of this invention is the use of a mathematical formula to correlate the response of the sensor to the concentration or dose of a particular CW agent, or group of CW agents, in the environment. Such mathematical models can be constructed for both reversible responses, which are concentration dependent, and irreversible responses, which are dosage dependent. In general, the responses to CW agents are irreversible due to the highly specific binding of the agent and the reactive additive.
In an additional aspect of the invention, a detection device is provided for displaying information indicative of a lethal dose of a CW agent in the environment. The device is preferably a dosimeter containing a sensor array, means for converting the measurable change in the detectable characteristic into a signal, means for comparing the signal with the response of the sensor array to a known or predetermined toxic substance, and means for displaying information on the concentration, dose or toxicity of the CW agent. The dosimeter can also include a microprocessor, data storage, and an alarm. The circuitry is compact and can be designed to fit into a credit-card sized monitor, which can be worn as a badge by the user.